One alternative is to identify the first and last letters only, but it isn't foolproof. Anything that involves repeatedly checking the answer sheet is probably going to slow them down even further. Chidren who have trouble with these questions do also seem to have trouble keeping their place as they work, and they are often the ones who have trouble locating right the answer box on the answer sheet as well. More checking of the answer sheet is unlikely to help.
By far the biggest problem with these questions is children solving them in the wrong direction and having to start again. Go to the second line and say to yourself "Am I being asked to find a word or a code?" If it is a WORD, go straight to the WORD on the top line and put your finger on it. Keep your finger there while you write the CODE above it. Then solve downwards. I find it helps to ignore the bit that says "what is the code for ...?" and "what does ... mean?" Just look at what you've got in front of you and decide what you are being asked for!
Then make sure that their non-writing finger is on the code to be solved so they don't lose their place. The pencil should be working along the alphabet (using dots, not loops, if they need to make any mark at all) for accurate counting.
There are two ways to attack these codes and different methods suit different children. Using this example:
If the code for PIANO is QKDRT, what is the code for VIOLIN?
Method 1 is to solve P to Q (+1) and immediately apply it to the V of VIOLIN, giving W, and so on for each letter, holding the required operation in memory.
Method 2 is to solve the sequence of operations first and only then apply it to the word VIOLIN. That invariably reveals a pattern, and it can save a lot of time. In this case the pattern is +1, +2, +3, ... By the time the third letter has been solved the pattern is obvious (subject to a quick glance to check that it doesn't revert to +1 again) and the child only needs to write out +4, +5 and then go straight on to finding the code for VIOLIN. Where the pattern is not a simple repetition of the same operation (e.g. +2, +2, +2) it may help to strike out each step as you go.
Some children can actually spot the pattern, hold it in their mind and apply it while using Method 1, but the great majority can't.
For long codes (6 or more letters) where the pattern is the same (e.g. +2, +2, +2) it is often only necessary to solve the first three letters and the last. That is where solving the pattern first comes into its own.
Also, for four letter codes, don't bother checking the answer sheet for "speed cheats" after 3 letters - it is quicker just to solve the code!
I hope that is clear and it speeds things up a bit.